Process of impregnating an assembled corrugated container board



May 8, 1962 R. c. MOKEE 3,033,708

PRocEss oF INPREGNATING AN ASSENBLED c'oRRUGATED CONTAINER BOARD Filed oct. 1, 195e MATERIAL LINER INVENTOR.

ROBERT C. MC KEE United States Patent Office 3,033,708 Patented May 8, 1962 3,033,708 PROCESS F MPREGNATHNG AN ASSEMBLED CORRUGATED CONTAINER EGARD Robert C. McKee, Appleton, Wis., assigner to The Institute of Paper Chemistry, Appleton, Wis., a corporation of Wisconsin Filed Oct. 1, 1958, Ser. No. 764,728 2 Claims. (Cl. 117-119.25)

This invention relates to improvements in corrugated board, and particularly to the manufacture of corrugated container board having impro-ved qualities of initial strength and the ability to retain such strength over a long period of time during which it is subjected to moisture or other atmospheric conditions which seriously affect the strength and durability of conventional corrugated container board.

The term corrugated container board is generally understood to mean an article in sheet form which includes a sheet of corrugated fibrous material in corrugated form, prepared from corrugating medium and usually called the corrugated medium, interposed between and adhesively united to a spaced pair of outer sheets of fibrous base material called liners Various types of paper or board may be employed for the liners and the corrugated medium. The liners, before assembly, have a weight of between 26 lbs. and 90 lbs. per 1,000 sq. ft., while the central sheet, called the corrugating medium before it is corrugated, generally Weighs between lbs. and 40 lbs. per 1,000 sq. ft. When the corrugating medium is corrugated, usually during the assembly of the three sheets, its length, depending upon the type of flute employed, is reduced by about thirty percent so that a corrugating medium after fluting or corrugating is about one-third greater in weight than the original medium before corrugating.

According to conventional practice, the three elements of the product are assembled on a corrugating machine. In the first step of the normal assembly operation, the corrugating medium is corrugated, and one of the liners is adhesively united to the crests on one side of the corrugated medium by some type of cementitious adhesive, for example, silicate of soda, or a starch-resin adhesive, and then subsequently the other liner is adhesively united to the opposite crests of the corrugated medium, usually with the same type of adhesive.

When the boxes or other containers made of corrugated container board are subjected to the conditions encountered during shipment or storage, atmospheric water vapor or aqueous liquids, either inside or outside of the container, tend to break down the adhesive joints between the elements of the board and also seriously affect the strength of the individual elements of the board of which the containers are constructed. This is particularly true in respect of the corrugated medium upon which the sheet assembly depends in substantial part for its resistance to bending or crushing stresses.

Various attempts have been made to reinforce or treat corrugated container board so as to improve its resistance to water or water vapor. However, great difficulty has been encountered in making satisfactory corrugated container board for use in the construction of containers, the contents of which contain free water or frozen products which shed moisture or give off water vapor during shipment or storage.

Although there are various ways in which the outer surfaces of the liners, or the liners themselves, may be coated or impregnated with a water-proofing or waterresisting agent Ibefore or after the assembly of the three elements is completed, it has not -been possible heretofore, so far as I am aware, to make the corrugated medium water-resistant. In view of the processes and equipment usually used in the manufacture of conventional corrugated container board, the treating of the corrugating medium before it is corrugated would introduce great difficulty from a practical standpoint. On the other hand, it was believed that any attempt to surface-treat the flutes of the corrugated medium after it is corrugated would be too expensive and troublesome to be practical from a commercial standpoint.

I have discovered a method of treating container board in such a manner as to effect an outstanding and unexpected improvement in the ability o-f the board to maintain a very large percentage of its original strength, notwithstanding that it may be subjected to the action of water or water vapor. If only water vapor is to be encountered, my improved method of manufacture permits adhesives weakened by water to be used for uniting together the elements of the corrugated container board. However, if liquid moisture is to be encountered, waterresistant adhesives should be used. In any case, my improved method necessitates the use of a substantially oleophobic adhesive.

As one feature of my invention, I have discovered that it is possible by using my process, to impregnate the corrugated medium of conventional corrugated container board, the elements of which are united together by an adhesive, which is not oil soluble, such as starch-resin. I accomplish this by applying a suitable coating of thermoplastic material to an exterior face of the board, and by then heating other portions of the coated Iboard for a suiiicient length of time at a temperature above the melting point of the material to cause the liquid wax to migrate into the board. If the temperature is high enough for a sufficient period of time, the wax is absorbed not only into the liner on which the coating is distributed, but it is also conducted through the seal or joint between the liner and the crest of the corrugated medium until it finally migrates through and into the corrugated medium. Although this effect may be observed by coating the board on one side only, the process obviously can be accelerated by coating the board on the outer surfaces of both liners.

I have found that although various types of relatively low melting thermoplastic materials may be employed for the purposes described, for purposes of efficiency and economy, I prefer to use a wax which is composed largely of paraflin having a melting point of between F. and F. The thermoplastic material must migrate in the container board at temperatures below the burning temperature of the board, and below the auto-ignition temperature of the material, and accordingly, should be relatively fluid at temperatures below about 300 F. Of course, the thermoplastic material should rbe solid at ambient temperatures to provide the desired strengthening of the container board.

Instead of using a Wax comprising one hundred percent parain, it may often be found advisable to use a wax which contains only 90 percent paraliin and the balance some other ingredient, for example, 10 percent microcrystalline wax. This additive tends to reduce the brittleness of the board after it has been treated and also tends to impart greater flexibility to the treated boardso that it may be more easily scored and folded. Also, it may be advisable possibly, in some cases, to employ small percentages of petrolatum (petroleum jelly) and polyethylene, instead of 10 percent of microcrystalline wax.

In the drawing accompanying this application, there is shown a fragmentary, greatly enlarged, cross section of a portion of a sheet of corrugated container board made in accordance with my invention.

I will now describe one way in which my invention has been successfully applied to the manufacture of corrugated container board used in the construction of containers for refrigerated foods such as (eviscerated) poultry which is wet-packed in iced or refrigerated boxes. Hereensayos tofore, because of the large amount of moisture in the contents of such boxes, it has not been feasible from a practical standpoint to make the boxes of corrugated container board. Wooden boxes (wire bound) have generally been employed.

As I have stated, it is not necessary in employing my invention, to use special water-proof or water-resistant adhesives in the manufacture of the board. However, if

`the board is to be subjected to liquid moisture, waterresistant adhesives such as starch-resin should be used. In either case, the board may be made in a conventional manner on a conventional machine. The rst step in the treating or impregnatiug or reinforcing of the board is to apply to at least one side, but preferably to both sides, of the container board, a coating of wax such as I have described. The amount of the coating is not critical; however, in order to obtain a satisfactory result, an amount of wax at least equal to percent of the weight of the untreated board should be used. It is not necessary, nor is it desirable, from the standpoint of expense, to use more than 60 to 80 percent of Wax.

In applying the wax, I have found it desirable for the board to be heated. Although other methods, such as rolls, etc., may be employed to apply the coating of heated liquid wax, I prefer to spray the liquid wax on both sides of the sheet, either simultaneously or successively, and if necessary, more than one coating may be applied in order to obtain the desired weight of coating on each side of the sheet. It is advisable to heat the wax to a temperature substantially above its normal melting point in order to enable the spraying devices to function satisfactorily.

The spraying can be effected as a continuous operation just after the container board leaves the double facer, i.e. after both liners have been applied to the corrugated medium, or the continuous sheet of board may be chopped off into lengths of the desired size before and not after the application of the wax coatings. In some cases, it may be advisable to feed the uncoated board upwardly between two applicators in order to obtain a suitable coating on each side of the board in one operation.

The lengths or sections of the board carrying coatings of the desired weight on their respective outer surfaces are then carefully stacked on a skid or pallet, the corrugations being oriented in the same direction until a pile of convenient height, four feet for example, has been built up. Then the stack or pile is trucked into a tunnel oven long enough to accommodate a suicient number of piles to keep up with the output of the other equipment. Heated air, meaning air heated to a temperature of about F. or more above the melting point of the wax, is then circulated through the oven and through the corrugations of the board so that each of the stacks of coated container board have been heated through and through to a temperature substantially above the melting point of the wax. That temperature is maintained for several hours. The tunnel may have sufficient capacity to receive the amount of board coated in a single shift of eight hours so that the entire batch of board can be heated for eight hours or more if necessary, and then removed so that the tunnel can be emptied in time for the next batch. The length of time necessary yfor the wax to migrate into the corrugated medium may be decreased by increasing the temperature in the oven. For example, at a temperature of 200 F., the boards may be impregnated inless than 2 hours with a good circulation of hot air through the boards. However, the temperature should not exceed the auto-ignition temperature of the wax, or the burning temperature of the board.

lIn order to reduce the time element in heating the relatively large stacks, it is desirable to arrange the stacks in the tunnel in such a way that the corrugations are all oriented in the same direction, and lengthwise of the tunnel. The hot air may be moved through the corrugations in the entire stack, and the stacks can be moved continuously through the tunnel counter current to the direction of ow of the air.

After a stack has been heated in the oven so that all parts of the stack are at a temperature well above the melting point of the wax, depending upon the composition of the wax and the temperature of the air and other conditions in the oven, the stack is removed from the oven. When the stack has been allowed to cool, the Wax has migrated into the corrugated medium and the adhesive connection between the medium and liner is protected. Accordingly, the operation is completed and the sheets of board can be removed. If the operation has been properly conducted according to the lmethod outlined herein, the separate sheets do not have any significant coating or residue of wax on the outsides of the liners. The coating has migrated into the sheet and it is found that the corrugated medium has distributed therethrough.

Wax impregnated corrugated container board, in which the elements are joined together by water-resistant adhesive, manufactured in accordance with the foregoing process can be scored, folded and made into boxes or tubes and can be used successfully in place of the wire bound wooden boxes which have yheretofore been employed, as containers for wet-pack food products or other products carrying a large percentage of water or moisture. Also, such containers, when used for any purpose, can be successfully employed in cases where as an incident to shipping or storage the containers cannot be protected from the weather. Adhesive weakened by water may be used to join the elements of the board together if the board is to be subjected only to water vapor.

The process as above outlined has been successfully used in treating corrugated container board in conventional weights. For example, I have successfully treated container board in which an adhesive, such as starchresin, was employed in connection with container board having liners each weighing 42 lbs. per 1,000 sq. ft. and a corrugating medium having a weight of 26 lbs. per 1,000 sq. ft. (before corrugating). Wax in the amount of 30 percent of the weight of the untreated board was used. In Ithis case, the blanks after scoring were pre-heated to 200 F., and then sprayed by a spray gun equipped with a hot spray tip, the air supply being heated to 150 F., using wax having a melting point of F., and using a pressure of 5 to l0 lbs. per square inch in the melted wax tank at a temperature of 180 F., and 45 lbs. per square inch of air at F. in the vair pressure tank.

Substantially the same procedure may be followed in improving or reinforcing conventional corrugated container board of any type or weight.

I am aware of various dipping processes for applying thermoplastic material to various boards. However, these processes require substantially greater amounts of such materials and do not result in uniform distribution of the materials through the board. Furthermore, such processes do not assure proper protection of the adhesive connection between the corrugated medium and the liners.

The various features of this invention which are believed to be new are set forth in the following claims.

I claim:

l. The process of impregnating an assembled corrugated container board having a fibrous outer liner element, a fibrous corrugated inner element, and oil-insoluble adhesive uniting said elements, which process comprises first distributing only over the outer surface of the fibrous outer liner element a limited amount of thermoplastic coating material, said vamount not being in excess of that which can be passed into said board to provide a coatingfree outer surface on said board, said thermoplastic material containing a major percentage of coating material which becomes uid at temperatures below the burning temperature of the board and which is solid at ambient temperature, and then subjecting the coated board in air to a temperature above the melting point of the fluidizable coating material so as to liquify the same, maintaining the board at said elevated temperature in air for a period of time suflicient to enable the uidized coating material to migrate from the outer surface of said liner element through the adhesive and into the corrugated inner element to provide a finished corrugated container board having Ia uniform distribution of said fluidizable coating material therethrough, the outer surface of said outer liner element being essentially `-free of a coating of said coating material.

2. The process of impregnating an `assembled corrugated container board having a pair of fibrous outer liner elements, a fibrous corrugated inner element and an oilinsoluble adhesive uniting said elements, which process comprises rst distributing over only the outer surface of each of the outer liner elements -a limited amount of coating Wax, vsaid amount not being in excess of that which can `be passed into said board to provide a Wax-free outer surface on said board, said Wax containing a major percentage of paratin, and then subjecting the coated board to heated air at a temperature above the melting point of the wax to liquify the Wax, and maintaining the board in heated air at said temperature for a period of time sufficient to enable the Wax to migrate from the outer surface of each of said liner elements through said adhesive yand into the corrugated inner element to provide a finished corrugated container board having a uniform distribution of said Wax therethrough, the outer surfaces of said liner elements being free of a coating of said wax, the amount of wax added to said board being from about 20 to 80 percent of the weight of said board before said impregnating.

References Cited in the file of this patent UNITED STATES PATENTS 329,743 Howe Nov. 3, 1885 395,645 Anderson Ian. 1, 1889 2,020,323 Rafton Nov. 12, 1935 2,576,278 Bode Nov. 27, 1951 2,723,923 Munters Nov. 15, 1955 FOREGN PATENTS 302,367 Great Britain Dec. 14, 1928 

1. THE PROCESS OF IMPREGNATING AND ASSEMBLED CORRUGATED CONTAINER BOARD HAVING A FIBROUS OUTER LINER ELEMENT, A FIBROUS CORRUGATED INNER ELEMENTS, AND OIL-INSOLUBLE ADHESIVE UNITING SAID ELEMENTS, WHICH PROCESS COMPRISES FIRST DISTRIBUTING ONLY OVER THE OUTER SURFACE OF THE FIBROUS OUTER LINER ELEMENT A LIMITED AMOUNT OF THERMOPLASTIC COATING MATERIAL, SAID AMOUNT NOT BEING IN EXCESS OF THAT FREE OUTER SURFACE ON SAID NOARD, SAID THERMOPLASTIC MATERIAL CONTAINING A MAJOR PERCENTAGE OF COATING MATERIAL WHICH BECOMES FLUID AT TEMPERATURES BELOW THE BURNING TEMPERATURE OF THE BOARD AND WHICH IS SOLID AT AMBIENT TEMPERATURE, AND THEN SUBJECTING THE COATED BOARD IN AIR TO A TEMPERATURE ABOVE THE MELTING POINT OF THE FLUIDIZABLE COATING MATERIAL SO AS TO LIQUIFY THE SAME, MAINTAINING THE BOARD AT SAID ELEVATED TEMPERATURE IN AIR FOR A PERIOD OF TIME SUFFICIENT TO ENABLE THE FLUIDIZED COATING MATERIAL TO MIGRATE FROM THE OUTER SURFACE OF SAID LINER ELEMENT THROUGH THE ADHESIVE AND INTO THE CORRUGATED INNER ELEMENT TO PROVIDE A FINISHED CORRUGATED CONTAINER BOARD HAVING A UNIFORM DISTRIBUTION OF SAID FLUIDIZABLE COATING MATERIAL THERETHROUGH, THE OUTER SURFACE OF SAID OUTER LINER ELEMENT BEING ESSENTIALLY FREE OF A COATING OF SAID COATING MATERIAL. 